Ultrasonic imaging of maternal and fetal tissues has greatly facilitated prenatal diagnosis and treatment. Among other uses in this field, ultrasound devices provide images which assist the physician in properly positioning a biopsy needle to perform amniocentesis, cordocentesis and transabdominal chorionic villus sampling.
While relatively broad, the ultrasound beam is generally flat. As a result, it has been difficult to keep the biopsy needle within the plane of the ultrasound beam. Once it escapes this plane, the needle will not be seen on the screen.
Various attempts have been made in the past to assist the physician in keeping the biopsy needle in view so that it may be properly guided into an aminotic sack, umbilical cord or placenta. U.S. Pat. Nos. 4,058,114 issued Nov. 15, 1977 to Soldner, 4,469,106 issued Sept. 4, 1984 to Harui, 4,576,175 issued Mar. 18, 1986 to Epstein and 4,582,061 issued Apr. 15, 1986 to Fry disclose needle guiding apparatus adapted for attachment to the probe or transducer of an ultrasonic device. While all of these devices provided some movement of the needle guide and needle relative to the transducer, such movement was significantly limited to prevent the needle from escaping the plane of the ultrasound beam. As a result, the physician was significantly hampered in positioning the needle prior to and during insertion, as well as in positioning the transducer once the needle was inserted into the mother's abdomen. As to this latter limitation, it is desirable to change the position of the ultrasound transducer once the needle has been inserted so that the physician is able to get a different, and perhaps improved, image of the fetus and target area for further positioning of the needle, as well as for additional diagnostic work.
It has also been found that the preferred angle of penetration of the biopsy needle for reaching the selected maternal or fetal tissue is approximately 45 degrees from an imaginary perpendicular plane projecting upwardly from the point of entry on the maternal abdomen. This angle is also ideal for obtaining a clear image of the needle on the viewing device. While this "ideal" angle of penetration could be achieved with some of the prior art devices disclosed in the above-identified patents, it was rather difficult to establish. The contour of the maternal abdomen is obviously not planar, was difficult to guage the proper angle without distracting the physician's attention from other, more critical, matters. Further, the ultrasound transducer is frequently held in non-perpendicular positions relative to the patient's abdomen, so it was a poor reference for guaging the penetration angle. The prior art needle guides were so constructed that, in order to obtain a suitable entrant angle for the biopsy needle, the angle of the transducer relative to the maternal abdomen had to be severely restricted.
Accordingly, the present inventor was confronted with the problems of devising a needle guide which would assist in keeping the needle within the plane of the ultrasonic beam, which would permit greater movement -- both of the biopsy needle prior to and during penetration and of the ultrasound transducer after penetration -- and which would make it easier to establish an optimal entrant angle for the biopsy needle.